Chemotherapy-induced ovarian failure is a growing public health problem with a major impact on quality of life. If individual toxicity of chemotherapy agents is known, patients can be counseled about the likelihood of ovarian damage and the need for fertility preservation. Moreover, if pharmacological ovarian protection strategies are developed, there will be no need for surgical interventions to preserve fertility, thus reducing concomitant risks and costs. The long term goal of this project is to improve our understanding of the risks of chemotherapy-induced ovarian failure by developing more accurate ovarian reserve assessment strategies and xenografting models, in addition to testing the effectiveness of pharmacological approaches to prevent chemotherapy induced damage to ovarian reserve in a xenograft model. The proposal focuses on breast cancer since it is the most common malignancy in young women. The specific aims are: (1) To determine the impact of commonly used breast cancer chemotherapy regimens on ovarian reserve using existing and emerging markers of the follicle population. Antral follicle counts, Inhibin-B, anti-mullerian hormone, and FSH/estradiol measurements will be obtained pre-and post-chemotherapy with a 2.0-4.5 year follow-up. (2) To measure the degree and determine the mechanism of damage caused by chemotherapeutics in human ovary. Immunodeficient mice with human ovarian tissue xenografts will be treated with commonly used and emerging agents, or the vehicle;changes in follicle density, as well as the extent of apoptotic follicular death as quantified by pre-cleaved caspase-3 expression will be used to measure the impact. An alternative mechanism of follicular loss by microvascular damage will also be investigated by quantitative vascular assessment methods and intra-vital dye injection. Moreover, we will investigate whether double strand DMA breaks are induced in the surviving primordial follicles by the analysis of ATM pathway proteins involved in DMA repair. (3) To determine whether a cell death inhibitor S1P protects against chemotherapy-induced damage to the primordial follicle pool, and ascertain whether the mechanism involves direct effects on primordial follicles via S1P receptors, or protection of ovarian microvasculature. These aims will be studied in short-and long-term xenograft models where not only the primordial follicle survival but the potential of S1P- protected primordial follicles to develop into antral stages and produce competent oocytes will be tested.